Spurred by the growth of the Internet, the trend towards telecommuting, the pace of telecommunication de-regulation, and various other factors, broadband access to the home has received an increasing amount of attention. In view of this attention, the telecommunication industry has been attempting to provide a communication system architecture and access scheme which is capable of delivering this range of new applications to the consumer, unfortunately, without much success.
One reason for this lack of success is the high bandwidth required to deliver these types of services. Bandwidth is a key limiting factor in determining the amount of information a system can transmit to a user at any one time. Bandwidth refers to the difference between the two limiting frequencies of a band expressed in Hertz (Hz). Although varying with application, effective broadband communication requires a bandwidth sufficient to permit a data rate in the range of several tens of mega-bits per second (Mbps).
Traditional wired communication systems using modems and a transmission medium such as twisted pair copper wire cannot currently achieve the data rates necessary to deliver high-speed applications due to bandwidth limitations. In an attempt to solve this bandwidth problem, the local exchange companies (LECs) have been engaged in planning and deploying hybrid fiber/co-ax (HFC), asymmetric digital subscriber line (ADSL) and switched digital video (SDV) networks. These wired-network approaches to providing high-speed access, however, require a substantial market penetration to keep per-subscriber costs at an acceptable level due to the high costs involved.
Similarly, traditional wireless systems such as narrowband cellular and Personal Communication Services (PCS) are bandwidth limited as well. This is especially true for wireless systems using lower frequencies (2-3 Ghz) where the bandwidth resource (i.e., frequency spectrum) is very limited. As an alternative, wireless solutions such as Multichannel Multipoint Distribution Service (MMDS) have become attractive for low take-rate scenarios, e.g., a market penetration of a few percent. The benefit of wireless systems for delivering high-speed applications is that they can be deployed rapidly without installation of local wired distribution networks. Even for these systems, however, there is a need for efficient use of the available bandwidth, in order to support the high data rate applications.
A traditional solution for increasing the bandwidth efficiency in wireless systems is through frequency reuse. Frequency reuse refers to reusing a common frequency band in different cells within the system. The concept of frequency reuse will be discussed in more detail with reference to FIGS. 1 and 2.
A typical wireless communication system includes a plurality of communication sites, such as mobile telephone switching office (MTSO), base stations, terminal stations, or any other site equipped with a radio transmitter and/or receiver.
FIG. 1 is a diagram of a typical wireless communication system. FIG. 1 shows a base station 20 in wireless communication with terminal stations 22. Base station 20 is usually connected to a fixed network, such as the public switched telephone network (PSTN) 24 or the Internet. Base station 20 could also be connected to other base stations, or connected to a MTSO and then PSTN 24 in the case of mobile systems. Terminal stations 22 can be either fixed or mobile.
Base station 20 communicates information to/from terminal stations 22 using radio signals transmitted over a range of carrier frequencies. Frequencies represent a finite natural resource, and are in high demand. Moreover, frequencies are heavily regulated by both Federal and State governments. Consequently, each cellular system has access to a very limited number of frequencies. Accordingly, wireless systems attempt to reuse frequencies in as many cells within the system as possible.
To accomplish this, a cellular system uses a frequency reuse pattern. A major factor in designing a frequency reuse pattern is the attempt to maximize system capacity while maintaining an acceptable signal-to-interference ratio (SIR). SIR refers to the ratio of the level of the received desired signal to the level of the received undesired signal. Most of the undesired signal is due to co-channel interference. Co-channel interference is interference due to the common use of the same frequency band by two different cells.
To implement frequency reuse, a cellular system takes the total frequency spectrum allotted to the system and divides it into K sets of frequencies. For example, if the system were allocated 50 MHZ of frequency spectrum, and there were 5 sets of frequencies (K=5), each set would include 10 MHZ worth of the 50 MHZ available to the system (assuming a uniform distribution). FIGS. 2(A) through 2(D) illustrate examples of frequency reuse patterns corresponding to K=4, 7, 12 and 19, respectively.
A cellular communication system has a number of communication sites located throughout the geographic area served by the system. As shown in FIGS. 2(A) through 2(D), this geographic area is organized into cells and/or sectors, with each cell typically containing a plurality of communication sites such as a base station and terminal stations. A cell is represented in FIGS. 2(A) through 2(D) as a hexagon. FIG. 2(A) shows a frequency reuse pattern where K=4. Cells are placed into groups of four, with each group employing one of the frequency sets 1 through 4 (the number within each cell in FIG. 2(A) represents a set of frequencies). This group of four cells is then repeated until the entire service area is covered. This same pattern is shown in FIGS. 2(B), 2(C) and 2(D) for groups of 7, 12 and 19 cells, respectively.
In order to increase spectrum efficiency, some cellular systems have employed multiple frequency reuse patterns within the same system. For example, U.S. Pat. No. 4,144,411 issued to Frenkiel on Mar. 13, 1979, teaches static reuse of frequencies in a system employing a miniature-sized overlay in each cell, with the miniature-sized overlay using the same type of reuse pattern as the large cell reuse pattern. The result is that the miniature-sized reuse pattern and the large-cell patterns are both on seven-cell repeat patterns. This is achieved through yet lower transmit powers and maintaining the same site spacing to cell radius as the large-cell. This concept is typically referred to as cell splitting.
An enhancement to Frenkiel is discussed in an article authored by Samuel W. Halpern entitled Reuse Partitioning in Cellular Systems, presented at the 33rd IEEE Vehicular Technology Conference on May 25-27, 1983 in Toronto, Ontario, Canada. The Halpern article sets forth a cellular system having multiple frequency reuse levels (or patterns) within a given geographical area. For example, a cluster of cells normally employing a seven-cell reuse pattern may simultaneously operate on a three-cell reuse pattern and a nine-cell reuse pattern. One set of frequencies is dedicated to the three-cell reuse pattern while another set of frequencies is dedicated to the nine-cell reuse pattern. Generally, the principle behind the Halpern system is to allow a degradation of carrier-to-interference (C/I) performance for those subscriber units that already have more than adequate C/I protection while providing greater C/I protection to those subscribers that require it. Therefore, a subscriber with the best received signal quality will be assigned to the set of channels for the three-cell reuse pattern since they are able to tolerate more co-channel interference than a subscriber whose signal quality is poorer. The subscriber having the poorer received signal quality is therefore assigned to a channel correspondent to the nine-cell reuse pattern.
The Halpern system, as well as the previous multiple frequency reuse partitioning systems, however, are unsatisfactory for a number of reasons. For example, in practice, the Halpern system permits only a small fraction of the total traffic to use the closer reuse pattern for the miniature-sized overlay, thus leading to little or no gain in capacity for the system. Further, the Halpern system is designed for circuit switched systems, and not for the modern packet switched systems. More specifically, circuit switched systems can tolerate a lot of measurement overhead and delay when connecting to the user. If the same techniques were applied to a packet switched system, however, these techniques would require several measurements before transmitting each packet. The overhead and delay introduced would be excessive, and therefore this real time measurement intensive method described in the Halpern reference would not be feasible. In fact, the Halpern method is designed for the conventional telephony system, and not packet switched systems in general.
Moreover, these previous systems were designed to do the reuse partitioning in the frequency domain, that is they were focused on dividing the total frequency bandwidth available to the system and allocating one portion of this total frequency bandwidth to one reuse pattern, and another portion to another reuse pattern.
Dividing the available frequency, however, limits the maximum data rate that can be provided to any single user or application by the system. Therefore, frequency reuse partitioning schemes are not suitable for supporting high data rate applications such as those envisioned for wireless broadband systems.
A specific implementation of frequency reuse partitioning is disclosed in U.S. Pat. No. 5,038,399 (the "Bruckert patent"). The Bruckert system is directed towards a mechanism for measuring various signal strengths from base stations and subscriber stations throughout the system, constructing a reuse level gradient, and using this gradient as a basis for switching between multiple frequency reuse patterns.
As with the Halpern system, the Bruckert system is unsatisfactory for a number of reasons. For example, the Bruckert patent is also targeted towards a circuit switched system and is not designed towards modern packet switched systems. Furthermore, the Bruckert patent describes a method for assigning different users to different reuse levels according to the "reuse level gradient," which is another way of stating the assignment is based upon different interference levels. In many instances, however, an integrated system providing different services to the same user may require different reuse levels due to their differing service requirements, even though they experience the same interference. The Bruckert patent fails to disclose how the quality of service (QoS) is maintained for each application using this method. In addition, the Bruckert patent fails to disclose any techniques for ensuring fairness among communication sites in terms of each site gaining access to the communication resource in a uniform manner. Finally, the Bruckert patent fails to disclose the use of multiple reuse patterns in the time domain, as with the previously discussed systems.
In view of the foregoing, it is clear that a substantial need exists for a scheduling scheme for use with a wireless communication system which provides high system throughput and guarantees QoS, while also being flexible, fair and demand driven